Electrical-motor-driven devices have become an integral part of life, and the availability of techniques for easily, efficiently, and reliably monitoring such motors is a necessity. A major cause of failure in polyphase electrical-motor-driven devices is the breakdown of the insulation which protects the motor windings. Service conditions, such as voltage imbalance, local high temperature ambient, etc., often have a significant negative effect on motor insulation. Such breakdown often necessitates expensive and time consuming repairs to the motor. Consequently, a need exists for a method to determine motor insulation conditions without de-energizing the motor.
Traditionally, motors have been protected from sudden failure by circuit breakers and temperature switches which respond to excessive current or temperature by shutting down the motor before it fails. However, these devices fail to disclose the cause of the problem or warn a user of the impending problem.
Other devices employ an AC signal to monitor insulation faults. However, these systems typically require complex electronics and coupling devices. Certain other systems utilize a simpler DC detection circuit. However, such systems typically can be used only when the motor is not operating.
In contrast to the prior art discussed above, U.S. Pat. No. 4,766,387, to Browne et al., discloses a method for monitoring the motor winding insulation resistance of a polyphase motor while the motor is running. The method is achieved by selectively disconnecting each winding from its respective phase power source and reconnecting that winding with an insulation measuring circuit. The insulation measuring circuit applies a test voltage to the winding so that the leakage resistance can be measured while the remaining windings continue to operate the motor.
Another method for monitoring faults in electrical installations is disclosed by Grunewald et al. in U.S. Pat. No. 4,897,607. The method is achieved by performing partial discharge measurements and high-frequency measurements at least at one location in the electrical installation, and optionally in all phases. The measurements are then compared with each other, and with the calibration signals, before conclusions are drawn regarding the location and the type of fault. Next, measurements at least at three measuring points are continuously and periodically performed while the electrical installation is in operation. A computer simulation of the electrical installation as a high-frequency network is then created, while simulating fault with the signal originating therefrom at the measuring points. The measurement values which indicate faults are then compared with the simulated signal values for different fault types and fault locations. Finally, the type and location of the faults are determined from the simulated values which best agree with the measurement values, and from the corresponding simulated fault.
Other fault detecting devices are disclosed in U.S. Pat. No. 5,126,678 to Williams and U.S. Pat. No. 4,377,784 to Saito et al.. The patent to Williams discloses a fault detector which monitors the phase difference between two line-to-line voltage of a generator, and compares the phase difference with a predetermined value or range of values to indicate a fault. The patent to Saito et al. discloses a fault detection apparatus which relies upon the pulsing magnetic flux waveform produced by a rotary machine to determine the presence or absence of faults.
In addition, symmetrical components analysis applied to non-symmetrical systems, as will be subsequently discussed in greater detail, has been used for some time. John R. Neuenswander, Modern Power Systems, International Textbook Co., 1971, pp. 155-166. However, the analysis has been limited in its application, and has been primarily used to determine faults in electrical transmission systems.
The prior art discussed above fails to disclose a convenient, efficient and reliable method for monitoring faults in an electrical motor driven device. The subject invention overcomes the shortcomings of the prior art by providing a method which utilizes a three phase equivalent circuit and symmetrical components analysis techniques to monitor real-world (unsymmetrical) faults in operating electrical motors.